This invention relates to nickel-based and cobalt-based superalloys, and, more particularly, to thermal barrier coating systems that protect such superalloys from oxidation and corrosion attack during operation.
One of the most demanding materials applications in current technology is found in turbine components used in aircraft jet engines. The higher the operating temperature of an engine, the greater its efficiency, and the more power it can produce from each gallon of fuel. There is therefore an incentive to operate such engines at as high a temperature as possible. One critical limitation on the operating temperature of an engine is the materials used in the hottest regions of the engine, such as gas turbine blades and vanes.
There has been an extraordinary amount of effort over the past 40 years to develop materials that can be used in high temperature engine applications. The currently most popular and successful of such materials are the nickel-based superalloys, which are alloys of nickel with additions of other elements such as chromium, cobalt, aluminum, tantalum, yttrium, and many others. Cobalt-based superalloys are also used in some situations. The compositions of these superalloys are carefully engineered to maintain their strength and other desirable properties even during use at the high temperature of engine operation, which is in the neighborhood of 2000.degree. F.
High operating temperatures can also be achieved by other techniques not related directly to the alloy compositions used in the components. For example, control of grain structures and use of single crystal components can result in improved properties. Cooling passages may be provided in the components, and cooling air passed through them.
In another approach that is the primary focus of the present invention, an insulating layer of a material having low thermal conductivity is deposited upon the component. This insulating layer, termed a thermal barrier coating or TBC, creates a thermal gradient from the surface of the superalloy component to the environment, so that the metallic component may be operated at a higher environmental or air temperature than otherwise might be possible. The insulating layer must protect the metallic structure from heat in this manner, be adherent to the superalloy substrate, and remain adherent through many cycles of heating to the operating temperature and then cooling back to ambient temperature when the engine is turned off. Because ceramics and metals have different coefficients of thermal expansion, cycles of heating and cooling tend to cause the ceramic coating to crack and spall off, which results in the superalloy being overheated in the area of the defect.
Ceramic coatings can act as thermal insulation for superalloy parts, but considerable care must be taken to ensure that the coating adheres well to the surface of the superalloy part, and remains adherent through many thermal cycles. To improve the adhesion and maintenance of adhesion of the ceramic coating, and to provide oxidation resistance for the underlying substrate, metallic bond coatings have been developed. The bond coatings are metallic compositions selected to form a stable, adherent surface oxide scale that protects the substrate from rapid oxidation. The combination of the ceramic coating, the metallic bond coating, and other constituents that might be present to protect the substrate is known as a thermal barrier coating (TBC) system.
In one approach to a thermal barrier coating system, a bond coat layer of metallic MCrAlY, where M is iron, nickel, or cobalt, or a mixture thereof, is first deposited upon the superalloy substrate. The surface of the bond coat is oxidized to produce a layer of protective predominantly aluminum oxide overlying the bond coat. The ceramic coating is then deposited on top of the aluminum oxide. If the ceramic coating is properly applied, it may have the form of separated columns extending perpendicular to the surface of the coating and the part, such as illustrated in FIG. 1 of each of U.S. Pat. Nos. 4,321,310 and 4,321,311.
Although such an approach may be useful, there is a continuing need for further improvements in thermal barrier coating systems. The present invention fulfills this need, and further provides related advantages.